This application claims foreign priority under 35 U.S.C. xc2xa7119 of Japanese Patent Application No. 11-326289, filed Nov. 17, 1999 and Japanese Patent Application No. 2000-272132, filed Sep. 7, 2000.
1. Field of the Invention
The present invention relates to an optical amplifier which uses a rare earth element-doped optical fiber applied to a wavelength division multiplexed (WDM) optical transmission system or the like. More specifically, the present invention is for obtaining an optical amplifier having small variation of gain with respect to changes in ambient temperature, and excellent temperature characteristics.
2. Description of the Related Art
Recently, optical amplifiers using optical fibers doped with a rare earth element such as erbium (Er), praseodymium (Pr), thulium (Tm) or neodymium (Nd); that is rare earth element-doped optical fibers, have been used for WDM-method optical transmission systems.
These optical amplifiers utilize the operation where an exciting light such as a laser beam is transmitted to a core of the rare earth element-doped optical fiber, the rare earth element ions are pumped with the exciting light to thereby form population inversion, and a signal light is input to the core in this state, to produce stimulated emission, and thereby optically amplify the input signal light.
Of the rare earth elements, particularly, with an optical amplifier using an optical fiber doped with erbium; that is an erbium-doped optical fiber (Erbium-doped Fiber Amplifier, EDFA), signal light of the 1550 nm band can be amplified with high gain and low noise. Hence application to high-speed, large-capacity, long-distance transmission systems is expected by means of high-density wavelength multiplexing.
FIG. 8 shows an example of a conventional optical amplifier of this type.
The signal light from an optical transmission line 10 passes through an optical isolator 11, and is input to an erbium-doped optical fiber 13 via an optical coupler 12. On the other hand, exciting light from an excitation light source 14 such as a semiconductor laser is input to the erbium-doped optical fiber 13 via the optical coupler 12. In the erbium-doped optical fiber 13, the signal light is optically amplified depending on the intensity of the exciting light, and the amplified signal light is output as an output signal light via an optical isolator 15, and transmitted to the optical transmission line 10.
With such an optical amplifier, it is known that the gain of the erbium-doped optical fiber 13 at the time of amplification decreases, with an increase in the environmental ambient temperature, and the decreased amount thereof differs depending on the wavelength.
In order to suppress a variation of gain due to temperature fluctuations, a method has been conventionally adopted in which the intensity of the output light from the erbium-doped optical fiber 13 or the ambient temperature of the optical amplifier is measured, and the input intensity of the exciting light from the excitation light source 14 such as a semiconductor laser, to the erbium-doped optical fiber 13 is electrically controlled base on the measurement.
Moreover, with a method disclosed in, for example, Japanese Unexamined Patent Application, First Publication No. Hei 9-293922, a measure is taken such that a temperature compensator having a loss-temperature characteristic which compensates for the variation in gain of the erbium-doped optical fiber 13 due to the temperature change, is inserted between the optical coupler 12 which combines the signal light and the exciting light, and the excitation light source 14.
With the former method however, since a sensor for detecting the output light, temperature or the like and an electronic circuit for control are required, there is a problem in that the entire apparatus becomes complicated and expensive. With the method which changes the output of the exciting light, there is a problem in that if the driven current of the semiconductor laser, being the excitation light source, is changed, the wavelength of the exciting light also changes, to thereby change the amplification characteristic itself of the erbium-doped optical fiber.
On the other hand, with the latter optical amplifier, it is difficult to obtain a temperature compensator having a desired loss-temperature characteristic, and it is also practically impossible to perform adjustment of the temperature characteristic.
It is therefore an object of the present invention to provide an optical amplifier which is hardly affected by changes in the ambient temperature, can obtain a stable amplification gain, and for which the gain-temperature characteristic can be easily adjusted.
Such an object can be achieved by inserting between an optical fiber for optical amplification which amplifies a signal light by photoexcitation and an excitation light source for supplying exciting light to the optical fiber for optical amplification, an optical fiber for temperature compensation which changes a supply quantity of the exciting light with temperature change, to thereby compensate for variations in gain with temperature change of the optical fiber for optical amplification.
As the optical fiber for temperature compensation, a rare earth element-doped optical fiber is preferable, and particularly, an erbium-doped optical fiber doped with 1000 to 2000 ppm of erbium is preferable.
Moreover, the intensity of the exciting light emitted from the optical fiber for temperature compensation can be finely adjusted by adjusting the length of the optical fiber for temperature compensation, to thereby easily obtain a desired temperature characteristic.